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Atlantic cod is a species that is affected by climate change, with some populations being exposed to higher temperatures than others. The two polymorphs of its major haemoglobin type (Hbl) show an inverse change in frequency along a latitudinal temperature dine in the North East Atlantic, which has been associated with differences in population performance and behavioural traits. An earlier study at the northern distribution limit of the species reported differential temperature sensitivities of red blood cell oxygen (O-2) affinity between the northern cold-water Hbl-2 polymorph and its southern. warm-water Hbl-1 counter-part, which has since widely been held as adaptive for the species across its distributional range. The present study critically re-examined this hypothesis by comparing the thermal sensitivity of O-2 binding in both purified Hbl polymorphs from the southern, high-temperature distribution limit of the species under controlled conditions of allosteric modifiers of Hb function. Contrary to the prevailing view, the O-2 affinity of the major Hbl polymorphs did not differ from each other under any of the tested conditions. Depending on pH and ATP concentration, the temperature-sensitive and temperature-insensitive Hb-O-2 affinity phenotypes - previously exclusively ascribed to Hbl-1 and Hbl-2, respectively - could be induced in both Hbl polymorphs. These results are the first to establish a molecular mechanism behind a reversed temperature dependence of red blood cell O-2 affinity in a non-endotherm fish and lay the basis for future studies on alternative mechanisms behind the differences in distribution, performance and behavioural traits associated with the different Hbl polymorphs of Atlantic cod.